A semiconductor laser typically comprises a body of material, generally group III-V compounds or alloys of such compounds, having a thin active layer between layers of opposite conductivity type. Constricted double heterostructure lasers, such as those disclosed by Botez in U.S. Pat. No. 4,347,486 and by Connolly et al. in U.S. Pat. No. 4,461,008 and entitled, TERRACED HETEROSTRUCTURE SEMICONDUCTOR LASER which are incorporated herein by reference, are capable of producing a single transverse (the direction perpendicular to the plane of the layers) and lateral (the direction in the plane of the layers and perpendicular to the direction of light propagation) mode, high power laser beam. These lasers have a guide layer adjacent to the active layer so that light generated in the active layer propagates mostly in the adjacent guide layer, thereby producing a much larger facet area from which light is emitted. The emission from one of the mirror facets of such a laser still occurs only over a small portion, typically in the order of several square micrometers (.mu.m), of a mirror facet of the device. The local power density is thus very high and may result in damage to the emitting mirror laser facet which can be either a slow, long term facet erosion or oxidation or it can be catastrophic in nature. To avoid either type of damage the laser output power density at the facet is held below the threshold at which such damage occurs. In addition a transparent coating, such as that disclosed by Ladany et al. in U.S. Pat. No. 4,178,564, incorporated herein by reference, may be placed on the emitting facet to increase the threshold at which the damage occurs. This combination of measures reduces the incidence of laser facet damage but the laser's inherent output power capability is still far from being fully used.
It has been suggested that catastrophic facet damage is caused by local heating of the mirror facet to a temperature close to the melting temperature of the material by absorption of the laser light. To reduce or eliminate this effect, semiconductor lasers have been fabricated in which the light absorbing active layer of the device does not extend to the facet. The region between the ends of the active layer and the facets is formed of a light transmissive material thereby eliminating the problem of light absorption at the facets. Such devices have shown an increase in the threshold powers at which the long term and catastrophic damage occur of between about five and ten times.
Such devices do not, however, provide lateral mode control particularly in the transparent regions adjacent to the mirror facets. Thus, the lateral mode character of the output light beam will depend upon the length of the transparent regions which will differ from device to device because of the inaccuracy present in the cleaving process used to form the facets.
Thus, it would be desirable to have a semiconductor laser having lateral mode control extending to a non-absorbing facet.